The present invention relates to an oil transfer unit for transferring oil between a stationary part and a rotating part. In particular, the following description will refer to an oil transfer unit for supplying lubricating oil to a rotating planet carrier of an epicyclic transmission in a turbine engine, but without losing in generality because of this explicit reference.
As is known, an epicyclic transmission comprises a sun gear, a ring gear and a plurality of planet gears, which are located between the sun gear and the ring gear and are supported by a carrier. A transmission of such a type is capable of transmitting the motion between coaxial shafts rotating at different speeds, and is very effective in providing such a function while maintaining small weight and volumes. Epicyclic transmissions are widely used in aeronautical turbine engines, to drive a fan (in so-called turbo-fan engines) or a propeller (in so-called turbo-propeller engines).
In most applications, the carrier is of static type and is coupled to a fixed frame of the engine by a flexible element. Under these conditions, the components supported by the carrier (the planet gears, possible rolling bearings, etc.) are lubricated without difficulty via ducts which are fixed with respect to the engine frame and to the carrier.
On the other hand, certain applications employ a rotating carrier, by way of example when the carrier is connected to a rotating driven shaft or when there is a need to continuously control the speed ratio between the sun gear and the ring gear or, alternatively, between the carrier and the ring gear. In particular, the configuration of the epicyclic transmission is called “planetary” when the ring gear is stationary and the carrier is rotating, and “differential” when all three elements, i.e. sun gear, ring gear and carrier, are rotating.
In these cases, an oil transfer unit is generally provided to transfer the lubricant oil in an efficient and reliable manner from a static part to a rotating part connected to the carrier. Such oil transfer units are generally known as “oil transfer bearings” or as “rotary unions”. The unit supplies oil under pressure into an annular chamber defined by a sleeve which is fixed to the carrier. From such annular chamber, the pressurized oil flows towards the components requiring lubrication.
The outer cylindrical surface of the sleeve has a radial passage arranged at the same axial position of the annular channel so as to put such channel into communication with the inner annular chamber. A minimum radial gap is provided between the inner cylindrical surfaces of the bearing and the outer cylindrical surface of the sleeve, to allow rotation of the sleeve and, in the meantime, to define a seal.
The amount of such radial gap is accurately determined in the design stage, so as to minimize leakages and therefore maximize the volumetric efficiency for the transfer of the oil. In the meantime, the mating cylindrical surfaces of the bearing and the sleeve have to be machined with a high precision level, to ensure the radial gap that has been defined at the design stage.
However, instable operating conditions can occur for the oil film at the radial gap between the above mentioned cylindrical surfaces. Such instable operating conditions alter the design setting established for the radial gap.
In particular, the radial width of the oil film between the cylindrical surfaces can decrease until reaching a zero value, so that a contact occurs between such cylindrical surfaces, while the latter are moving one with respect to the other. These possible contacts are a severe source of wear and can even cause seizing of the bearing onto the sleeve.